You will want to actually count them. Start counting "out loud" in your mind using your "inner voice", then make that voice whisper, so as to not give to much attention to it. - It is rather hard to explain.

Using images, or mental pictures is NOT the way to go. Since hypnogogic imagery is a mix between the minds-eye and the actual eye, once they start to occur, you will begin to have problems. Shorty you either forget you were counting, or be unable to picture the numbers.

The purpose of counting is to keep your Logical part of your mind awake. However, it also aids in having something to focus on while you get deeper into your sleep. With that said, Im not sure if the method you use has the same effect on the Logic part of your mind/brain. - When counting you eventually want it to become second nature, meaning, your mind will keep counting, even if you forget about it; be to the point where you can think about X and not lose track of where you were or what number(s) you were on.

When I said "Start from 1 then go to 100, once at 100 go to 1, Repeat." I meant this: Example:
1, 2, 3, .., 98, 99, 100, 99, 98, .., 3, 2, 1, 2, 3, 4, 5, .., 98, 99, 100, 99, 98, 97, .. .. ..

I have found this method to be much more effective for keeping tack of location while remembering to count. The reason is doing a progressive-digressive count keeps the numbers continually flowing, rather then just restarting, it also creates a loop that is 2x as long then a general 1-100 sequence. - The reason to stop at 100 is; With the word(s) "One hundred and X" having to be repeated, it causes a person to lose focus on the numbers themselves. Another reason is when you begin to see images, hear sounds, etc, your mind will begin to place words and other images to what your just saw and heard. (ex. You are at the number "One hundred and three", then you see your self in class tearing a piece of paper and you think "a ton of paper just tore", which sounds extremely close to your next count, thus causing confusion of where you are, sometimes even making you forget to keep counting....